Long Term Changes in Stand Structure and Biomass Production in Short Rotation Willow Coppice

Nordh, N.-E. 2005 Long term changes in stand structure and biomass production in short rotation willow coppice. Doctor’s dissertation. ISSN 1652-6880, ISBN 91-576-6919-8. Short rotation willow coppice (SRWC) is a recently commercialised agricultural crop in Sweden, producing biomass for energy. The expected lifespan of a SRWC plantation is more than 20 years, or at least 5 harvests. Consequently, understanding of long term stand development in relation to management, plant material and site characteristics is of great importance. The focus of this thesis is on SRWC biomass production during later cutting cycles. Long term development and dynamics have been studied during three cutting cycles by means of annual census of individual stools, scaled up to stand level. Methods for non-destructive biomass assessment, by means of allometric relations, have been developed and validated for different clones. A method for predicting willow shoot growth in field trials, under different nutrient and water conditions, based on characteristics found in pot-grown plants under corresponding conditions, has also been investigated. The best non-destructive method for assessing shoot weight of clones with bow-shaped and branching stems was found to be shoot dry weight related to the sum of the cross sectional areas of all shoots. In straight, un-branched clones, the measuring height could be elevated to 105 cm above stem base without loosing precision. A comparison of a destructive and a non-destructive method, applied on 12 different clones, showed a mean deviation of 2.5%. Stool mortality during the 1 cutting cycle was non-density dependent. Biomass production increased in the 2 cutting cycle but high density dependent stool mortality at the end of the 2 cutting cycle negatively influenced the production in the 3 cutting cycle, which was lower than both the 1 and 2 cutting cycle. A stool size hierarchy was established early and prevailed through all cutting cycles. Stool mortality occurred mainly among small stools. In the beginning of the 4 cutting cycle, biomass production stabilised due to compensatory growth of the remaining stools. Total leaf area and total nitrogen pool of pot-grown plants were good clone-specific characters for predicting shoot biomass growth in the field during the first cutting cycle and may be used for shortening the time needed to characterise new clones. The results suggest that sustainability in SRWC systems is enhanced by matching clone and site and adapting fertilisation and harvest timing to actual stand development.